Raisin bun-type composite spheres of silica and semiconductor nanocrystals

CdTe nanocrystals capped with 1-mercapto-2,3-propandiol, CdSe nanocrystals capped with sodium citrate, and core-shell CdSe/CdS nanocrystals capped with sodium citrate were synthesized in aqueous solutions, and their surface was modified by 3-mercaptopropyltrimethoxysilane (MPS) in water-ethanol mixtures. By addition of sodium silicate, raisin bun-type composite particles were formed, with either CdTe, CdSe, or CdSe/CdS nanocrystals being homogeneously incorporated as multiple cores into silica spheres of 40-80 nm size, accompanied by some alteration of optical properties of the nanoparticles and, in particular, the reduction of the luminescence quantum yield. Further, growth of larger silica spheres (100-700 nm) can be performed by the Stober technique using either MPS-modified semiconductor nanocrystals or raisin bun-type composite particles as seeds, which gives semiconductor-doped silica globules of desirable sizes in the submicrometer range. The composite spheres can be used as building blocks for 3D colloidal crystals, prepared in this study for CdS/CdSe-doped 250 nm silica colloid. The shift of the photonic band gap to the red was observed in photonic crystals made of nanoparticles-doped silica due to the high refractive index of the semiconductors.